HUBBLE SPACE TELESCOPE OBSERVATIONS OF THE DWARF NOVA Z CHAMAELEONTIS THROUGH TWO ERUPTION CYCLES

摘要

We have obtained the first high-speed photometry of the eclipsing dwarf nova Z Cha at ultraviolet wavelengths with the Hubble Space Telescope. We observed the eclipse roughly every 4 days over two cycles of the normal eruptions of Z Cha, giving a uniquely complete coverage of its outburst cycle. The accretion disk dominated the ultraviolet light curve of Z Cha at the peak of an eruption; the white dwarf, the bright spot on the edge of the disk, and the boundary layer were all invisible. We were able to obtain an axisymmetric map of the accretion disk at this time only by adopting a flared disk with an opening angle of ～ 8°. The run of brightness temperature with radius in the disk at the peak of the eruption was too flat to be consistent with a steady state, optically thick accretion disk. The local rate of mass flow through the disk was ～5 x 10~(-10) solar mass yr~(-1) near the center of the disk and ～5 x 10~(-9) solar mass yr~(-1) near the outer edge. The white dwarf, the accretion disk, and the boundary layer were all significant contributors to the ultraviolet flux on the descending branches of the eruptions. The temperature of the white dwarf during decline was 18,300 K < T_(wd) < 21,800 K, which is significantly greater than at minimum light.